Conformation in dogs refers solely to the externally visible details of a dog's structure and appearance, as defined in detail by each dog breed's written breed standard.[1][2][3] A dog that conforms to most of the items of description in its individual breed standard is said to have good conformation. Unlike equine conformation, there are no fixed rules for dog conformation, as dogs are the most variable in appearance of any animals ("Phenotypic variation among dog breeds, whether it be in size, shape, or behavior, is greater than for any other animal"[4]). Instead, conformation in dogs is based on the dog type from which the breed developed, along with many details that have been added to the breed standard for purposes of differentiation from other breeds, for working reasons, or for enhancing the beauty of the animals from the viewpoint of the fanciers who wrote the breed standards.[5][6]